A magnetic head of a hard disk drive, a typical disk device as an external storage of a computer, follows a target track on a magnetic disk surface that is rotating and writes and reads data to and from the track. Therefore, the hard disk drive must have a control mechanism for correctly positioning the magnetic head onto the target track. The track width on the magnetic disk has become smaller in recent years to increase the storage capacity and improvement in positioning accuracy and positioning speed of the head has become more and more necessary.
A positioning control system of the magnetic head of a hard disk drive which is an example to which the present invention is directed, includes two kinds of controls, that is, seek control for moving the magnetic head to a target track and following control for allowing the magnetic head to accurately follow the same track.
Causes that deteriorate positioning accuracy of the head include disk oscillation resulting from fluid forces called “disk flutter”, oscillation of a position signal that is in synchronism with the revolution of the disk and oscillation of a mechanical system (rotation-synchronous oscillation). These oscillations appear as specific frequencies such as a frequency that is integer multiples of the rotating frequency of the disk and a natural frequency of the mechanical system. To improve head positioning accuracy even when such oscillations exist, it is effective to design a positioning control system of the head by using a digital filter (resonance filter) having a resonance point at that frequency. To prevent the occurrence of transient response, such a resonance filter stops calculation during the seek operation of the head and starts the calculating operation after the head comes in the proximity of the target track. Therefore, the technology described above needs a long time for suppressing the disturbance after the head reaches the proximity of the target.
One of the prior art technologies for solving such a problem is the technology described in U.S. Pat. No. 5,608,586 (patent document 1). This prior art relates to the technology that allows a resonance filter to follow at a high speed an oscillation at a specific frequency without transient response, continues the calculation of the resonance filter during the seek operation by setting the input to the resonance filter to 0 and allows free oscillation of the resonance characteristics realized by the resonance filter.